All data are presented as the mean±s.d. Fatigue index (in %) is the maximal tetanic tension (P0) after stimulation for 5 minutes at 1 Hz. Six animals in each group were assayed for mass, twitch and tetanic tension, fatigue and resting length. Three animals in each group were diameter, labeling was done using Evans Blue dye, central nuclei, plasma creatine kinase, and labeling for slow-twitch myofibers. MHC-slow, the percent myofibers labeled for the slow-twitch isoform of the myosin heavy chain. *P<0.001 and **P<0.05, statistically significant differences between K19-/- and control.